The invention relates to a method of sealing a joint and, in particular, to a method of sealing an interfay joint during the manufacture of an aircraft.
In the manufacture of an aircraft it is necessary to provide fluid-tight sealing in many of the joints between components, for example, to prevent fuel leakage from fuel tanks or air leakage from pressurised cabins and to prevent water ingress into joints and consequent corrosion in those joints. Joints between components of an airframe are known as interfay joints.
The requirements for sealants used in aircraft include resistance to fuel and water, chemical compatibility with the metals and coatings used in the aircraft components and reliable maintenance of sealing performance over a wide temperature range and over the lifetime of the aircraft.
Polysulphide sealants are known for use in a range of sealing applications and, in particular, are known for use in sealing airframe interfay joints. The polysulphide sealant is typically cured via an oxidative mechanism promoted by a transition metal oxide, for example, manganese dioxide or dichromate compounds or via an organic chemical reaction, for example, epoxidation or condensation esterification. In general, the polysulphide sealants are currently used in aircraft manufacture are two-component systems, with the mixing of the components being done shortly before use of the sealant (one-component polysulphide sealants are known but do not, in general, meet the demanding requirements for use in aircraft).
In aircraft manufacture, the components to be assembled into the joint will usually be of lightweight aluminium alloy and will usually be painted prior to assembly. The joining or mating faces are first prepared by cleaning with a suitable solvent, light abrasion and wiping dry. Freshly-mixed sealant is then applied directly on to at least one mating face and the components are offered together and then fastened with bolts or the like. That known method of joint assembly suffers from a number of disadvantages, including:                the premixing of the sealant components prior to use is labour-intensive, messy and requires accurate control and measurement of the levels of curing promoter in order to ensure that the period during which the sealant remains workable (the “work life”) is sufficient for the planned joint assembly;        the work life and the time required to cure the sealant both depend, inter alia, on the local conditions, in particular, humidity and temperature;        application of the correct amount of liquid sealant requires skill and experience;        the joint must not be disturbed until the sealant is cured which in practice may mean that no further work can be carried out on the components for a period of several days; and        during tightening of the bolts, the liquid sealant is squeezed away from the immediate vicinity of each bolt thereby allowing direct contact of the components such that subsequent relative movement of the components during the lifetime of the aircraft can cause the paint to rub off, leaving the joint vulnerable to corrosion (this is known as “fretting”).        
Our British patent application No. 0329891.6 “A Sealing Material” filed on 23 Dec. 2003, discloses solid sheet sealant materials comprising a cured polysulphide sealant and a reinforcing element, for example, glass fibre. In one process, the mixed polysulphide sealant is spread over the glass fibre and is then pressed in a press while the polysulphide cures to produce a sheet or film of cured polysulphide/glass fibre composite. In use, the sheet is cut into shape and fitted like a gasket in between the mating surfaces of two components. The components are then fastened together with bolts or the like to form the joint. As the fasteners are tightened, the metal of the components in the immediate vicinity of each fastener distorts such that in those areas the mating surfaces are brought closer together. A solid sealing material or gasket located between those surfaces will, however, reach a minimum thickness beyond which it cannot be compressed further, in contrast to the conventional liquid sealants which are entirely, or almost entirely, squeezed out of the vicinity of each fastener.
It is highly desirable for the sealing material to seal effectively in both the areas of high compression around the fasteners and in the areas of lower compression in the “quilted” pockets between the fasteners. In general, using an increased thickness of sealing material will improve the sealing action. However, using such an increased thickness of sealing material increases the weight of the aircraft, involves additional cost and reduces any contribution to the stiffness of the joint made by the sealing material.
The behaviour under compression of a particular solid sealing material depends upon the physical and chemical nature of the material but also depends upon the dimensions of the material and the effects of any fillers, reinforcing elements or voids in the material. Thus, for a sealant of a particular chemical type, such as a polysulphide sealant, the actual behaviour under compression shown by different sealing materials including that sealant may vary over a wide range.
For the above reasons, there is a need to improve the process of identifying the optimum sealing material and the thickness of that sealing material to be used in a joint.